A proposed three strikes law would have done nothing to prevent some of New Zealand's most notorious killings, critics say.

The new legislation, to be introduced with the Sentencing and Parole Reform Bill, will see a criminal convicted of a third qualifying offence jailed for the maximum term, without the possibility of parole.

Under the law, a life sentence - imposed for murder and manslaughter - would mean just that.

But none of the country's 20 worst murderers - those handed among the longest jail sentences in New Zealand history - would have been stopped by the proposed legislation, the Weekend Herald has learned.

And that includes four of the worst:

William Bell, who is serving a minimum 30 years for the Mt Wellington triple murders of 2001, double killer Graeme Burton, convicted rapist and murderer Liam Reid, and prison-van killer George Charlie Baker.

The Corrections Department has confirmed none of them - despite extensive criminal histories, including in the case of Burton more than 100 previous convictions - had committed earlier offences that would have put them in the three strikes category.

While supporters such as the Sensible Sentencing Trust have welcomed the three strikes law, opponents have criticised it as a political "gimmick" that will achieve little.

Labour leader Phil Goff said the three strikes law would lock up 12 extra criminals each year for the next five years.

"That's a drop in the bucket. If the new law wouldn't have touched on the people you're talking about, that shows it is more about the rhetoric, rather than ... making people safer."

Mr Goff said more money should be spent to help stop young offenders graduating to hardened criminals.

"But that's less sexy and headline-grabbing than pretending to have the toughest law ever."

Kim Workman, director of penal reform group Rethinking Crime and Punishment, said the longer sentences would not deter violent criminals.

"The sort of offenders we are talking about don't think once, let alone twice."

Figures released by Corrections Minister Judith Collins said the new law would add 56 inmates to the prison population after five years, 142 after 10 years and 288 after 20 years.

Judges now have discretion to apply minimum non-parole sentences for murder. The new law will remove that discretion in all but exceptional cases in which the judge believes that to sentence for the maximum period would be "manifestly unjust".

Mrs Collins has said the law "will be harsh - but only for the small number of people in our community who show continued disregard for the law and contempt for society".

LIFE-SAVERS

The proposed three-strikes law would have saved murder victims including:

Urban Hoglin and Heidi Paakkonen
The Swedish tourists, aged 23 and 21, were holidaying in the Coromandel in 1989 when they went missing. David Tamihere was found guilty of their murders in 1990 but still claims to be innocent - a barrier to a successful parole bid. At the time of the murders, Tamihere had skipped bail on a 1986 rape and was on the run. He also had a conviction for the 1972 manslaughter of an Auckland prostitute.

Jo-Anne van Duyvenbooden
The Tauranga sex worker was shot in her sleep by ex-boyfriend James Henry Wilson, who was on bail for manufacturing P. The president of the Filthy Few Tauranga chapter, "Little Willy" is considered one of New Zealand's most callous criminals whose conviction for murder in 2000 was the culmination of a 25-year life of crime. He bashed a tavern manager with a shotgun in an aggravated robbery in 1983, then dripped acid on an Opotiki couple in a home invasion in 1992. He also indecently assaulted the bound and blindfolded woman.

Sarah Curry
The 8-year-old Invercargill girl was found in a reserve after being raped and murdered by Peter Davis in 1992. Davis had a 1987 conviction for rape, but was released early from his five-year sentence. He is due before the Parole Board again this year.

Louisa Damodran
The 6-year-old Christchurch girl was abducted and murdered by Peter Holdem in 1986. Holdem had just been released from jail for the attempted rape of a 10-year-old girl and had a huge number of convictions for sex offending against young girls, burglary and dishonesty offences.

Maria Copley
The 41-year-old mother was bashed to death by David William Taylor in Wellington's Basin Reserve in 1997. He was stoned on cannabis and drunk on 18 cans of beer when he killed her, after spending 18 of his 42 years in prison for violent offences.

Leonie Newman
The Hamilton woman was stabbed 28 times by ex-partner Leon Colin Wilson in 1995. He had previously tattooed "Property of Leon" across her face and had a string of convictions.

Terry Norton
The Wanganui man was found dead in his car, which was submerged in the Whanganui River in 1992, after a confrontation with Black Power. Gang member Hukamolo Saiffiti, 23, was convicted of his murder. He had a number of previous violence offences.